In recent years, optical modulations, such as differential phase shift keying (DPSK) and differential quadrature phase shift keying (DQPSK), have attracted attention as technologies enabling high bit-rate optical transmission of 40 Gb/s or more.
DQPSK has advantages in long-distance transmission, dense multiplexing/high capacity transmission and design performance, usability and the like, compared to publicly known optical modulations including general non-return-to-zero (NRZ) modulation, carrier suppressed return-to-zero modulation (CS-RZ) and return-to-zero differential phase shift keying. Note that DQPSK in the present specification includes RZ-DQPSK, in which a DQPSK signal is return-to-zero pulsed, and carrier-suppressed RZ-DQPSK.
A brief description is given below of a DQPSK-applied optical transmitter and a DQPSK-applied optical receiver. As a DQPSK-applied optical transmitter, one having a basic configuration illustrated in FIG. 1, for example, is known.
In the optical transmitter, continuous light emitted from an optical source 3-1, such as a distributed feedback laser (DFB), is divided into two elements, and one of the elements is input to a first phase modulator (PM) 3-2 and the other is input to a second phase modulator (PM) 3-3 and a phase converter 3-4.
The phase modulators (PM) 3-2 and 3-3 are independently driven according to modulated signals ρk and ηk, respectively, generated by a precoder 3-5 using two data signals Ik and Qk, and selectively change the phase of each input light element to 0 or π [rad]. As compared to the light passing through the light path of the phase modulator (PM) 3-2, a phase difference of π/2 is given to the light passing through the light path of the phase modulator (PM) 3-3 by the phase converter 3-4.
Accordingly, while the light output from the light path of the phase modulator (PM) 3-2 is a light signal created by shifting the phase of the light from the optical source 3-1 by 0 or π, the light output from the light path of the phase modulator (PM) 3-3 is a light signal created by deviating the phase of the light from the optical source 3-1 by π/2 or 3π/2. Then, by multiplexing the light output from these light paths, it is possible to generate DQPSK signal light with four different phases, π/4, 3π/4, 5π/4 and 7π/4.
The bit rate of the DQPSK signal light is twice the bit rate of the data signals Ik and Qk processed by the precoder 3-5. Therefore, in order to transmit DQPSK signal light of 40 Gb/s, for example, the individual phase modulators (PM) 3-2 and 3-3 are driven using data signals of 20 Gb/s.
The DQPSK signal light is RZ-pulsed (return-to-zero pulsed) by an intensity modulator 3-7 driven by a clock signal having a duty ratio of 50%, which clock signal is in sync with the data signals, whereby RZ-DQPSK signal light is generated. In addition, by setting the duty ratio of the clock signal to 66% or the like, CSRZ-DQPSK (carrier suppression return-to-zero DQPSK) signal light is generated.
The precoder 3-5 performs logical operations of Equations (2) below, which are obtained by developing and sorting Equations (1).
                    [                  Equations          ⁢                                          ⁢          1          ⁢                                          ⁢          and          ⁢                                          ⁢          Equations          ⁢                                          ⁢          2                ]                                                                                                                                  ρ                  k                                =                                                                                                                                                                                        (                                                                                                I                                  k                                                                ⊕                                                                  ρ                                                                      k                                    -                                    1                                                                                                                              )                                                        _                                                    ⁢                                                      (                                                                                          I                                k                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                    ⁢                                                      (                                                                                          ρ                                                                  k                                  -                                  1                                                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                                          +                                                                                                                                                                                                      (                                                                                          Q                                k                                                            ⊕                                                              ρ                                                                  k                                  -                                  1                                                                                                                      )                                                    _                                                ⁢                                                  (                                                                                    Q                              k                                                        ⊕                                                                                          η                                                                  k                                  -                                  1                                                                                            _                                                                                )                                                ⁢                                                  (                                                                                    ρ                                                              k                                -                                1                                                                                      ⊕                                                                                          η                                                                  k                                  -                                  1                                                                                            _                                                                                )                                                                                                                                                                                                              η                  k                                =                                                                                                                                                                                        (                                                                                                Q                                  k                                                                ⊕                                                                  η                                                                      k                                    -                                    1                                                                                                                              )                                                        _                                                    ⁢                                                      (                                                                                          Q                                k                                                            ⊕                                                              ρ                                                                  k                                  -                                  1                                                                                                                      )                                                    ⁢                                                      (                                                                                          ρ                                                                  k                                  -                                  1                                                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                                          +                                                                                                                                                                                                      (                                                                                          I                                k                                                            ⊕                                                              η                                                                  k                                  -                                  1                                                                                                                      )                                                    _                                                ⁢                                                  (                                                                                    I                              k                                                        ⊕                                                                                          ρ                                                                  k                                  -                                  1                                                                                            _                                                                                )                                                ⁢                                                  (                                                                                                                    ρ                                                                  k                                  -                                  1                                                                                            _                                                        ⊕                                                          η                                                              k                                -                                1                                                                                                              )                                                                                                                                                        }                            (        1        )                                                                                                      ρ                  k                                =                                                                            Q                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                          η                                              k                        -                        1                                                                              +                                                            I                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                                                  η                                                  k                          -                          1                                                                    _                                                        +                                                                                                              I                          k                                                ⁢                                                  ρ                                                      k                            -                            1                                                                                              _                                        ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    Q                        k                                            ⁢                                              ρ                                                  k                          -                          1                                                                    ⁢                                              η                                                  k                          -                          1                                                                                      _                                                                                                                                            η                  k                                =                                                                            I                      k                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    Q                        k                                            _                                        ⁢                                          ρ                                              k                        -                        1                                                              ⁢                                                                  η                                                  k                          -                          1                                                                    _                                                        +                                                            Q                      k                                        ⁢                                                                  ρ                                                  k                          -                          1                                                                    _                                        ⁢                                          η                                              k                        -                        1                                                                              +                                                                                    I                        k                                            ⁢                                              ρ                                                  k                          -                          1                                                                    ⁢                                              η                                                  k                          -                          1                                                                                      _                                                                                      }                            (        2        )            
In Equations (1) and (2) above, Ik and Qk are logical values (1 or 0) of the k-th clock cycle signals (i.e. signals in the k-th clock cycle) to be input to the precoder 3-5 illustrated in FIG. 1, and ρk and ηk are logical values (1 or 0) of the k-th clock cycle signals output from the precoder 3-5. The suffix k−1 indicates that a value to which the suffix is attached is a logical value in the clock cycle immediately preceding the k-th clock cycle.
According to the configuration example illustrated in FIG. 1, in order to implement the logical operations, the output signals ρk and ηk of the precoder 3-5 are fed back to the input of the precoder 3-5 via delay elements 3-6, each of which applies a one-symbol time delay τ. A specific configuration of the precoder 3-5 is illustrated in FIG. 2. With reference to FIG. 2, logical values one symbol before the logical values ρk and ηk are fed back via delay elements D which apply the time delay τ.
In addition, a parallel precoder circuit having, for example, a configuration illustrated in FIG. 3 is known as a mechanism for implementing the logical operations of the precoder 3-5 of FIG. 2 at low speed (see Japanese Laid-open Patent Application Publication No. 2006-245647).
The parallel precoder circuit of FIG. 3 is configured in the following way. A parallel precoder unit 4 is provided in which eight precoders, each implementing the logical operations of Equations (2) above, are disposed parallel to each other. The logical operations are simultaneously performed on sequentially taken-out eight symbols of each data signal I and Q in a parallel fashion. In the next cycle, the logical operations are again simultaneously performed on the subsequent eight symbols of each data signal I and Q in a parallel fashion. The same operation process is subsequently repeated.
That is, the following eight precoders are provided: a first precoder 4-1 for performing the logical operations on the 8kth data signals I8k and Q8k; a second precoder 4-2 for performing the logical operations on the 8k+1st data signals I8k+1 and Q8k+1; a third precoder (not shown) for performing the logical operations on the 8k+2nd data signals I8k+2 and Q8k+2; a fourth precoder (not shown) for performing the logical operations on the 8k+3rd data signals I8k+3 and Q8k+3; a fifth precoder (not shown) for performing the logical operations on the 8k+4th data signals I8k+4 and Q8k+4; a sixth precoder (not shown) for performing the logical operations on the 8k+5th data signals I8k+5 and Q8k+5; a seventh precoder 4-7 for performing the logical operations on the 8k+6th data signals I8k+6 and Q8k+6; and an eighth precoder 4-8 for performing the logical operations on the 8k+7th data signals I8k+7 and Q8k+7. Operation results ρ8k and η8k through ρ8k+7 and η8k+7 of the individual precoders 4-1 through 4-8 are synchronized with each other by eight flip-flops that make up a re-timing unit 5, and are output as DQPSK modulation signals.
A connection is established in such a manner that each set of the operation results ρ8k and η8k through ρ8k+7 and η8k+7 is input to the next-level precoder as operation results ρ8k−1 and η8k−1 via the re-timing unit 5. That is, the operation results of ρ8k and η8k of the first precoder 4-1 are input to the next second precoder 4-2; the operation results of ρ8k−1 and η8k+1 of the second precoder 4-2 are input to the next third precoder (not shown); the operation results of ρ8k+2 and η8k+2 of the third precoder are input to the next fourth precoder (not shown); the operation results of ρ8k+3 and η8k+3 of the fourth precoder are input to the next fifth precoder (not shown); the operation results of ρ8k+4 and η8k+4 of the fifth precoder are input to the next sixth precoder (not shown); the operation results of ρ8k+5 and η8k+5 of the sixth precoder are input to the next seventh precoder 4-7; the operation results of ρ8k+6 and η8k+6 of the seventh precoder 4-7 are input to the next eighth precoder 4-8; and the operation results of ρ8k+7 and η8k+7 of the eighth precoder 4-8 are input to the next first precoder 4-1 (i.e. the next-level precoder in the next logical operation cycle).
The logical operations performed in the first through eighth precoders 4-1 through 4-8 are expressed by Equations (3) below.
                    [                  Equations          ⁢                                          ⁢          3                ]                                                                                                                                  ρ                                      8                    ⁢                    k                                                  =                                                                                                                                                          Q                                                          8                              ⁢                              k                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            -                              1                                                                                ⁢                                                      η                                                                                          8                                ⁢                                k                                                            -                              1                                                                                                      +                                                                              I                                                          8                              ⁢                              k                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            -                              1                                                                                ⁢                                                                                    η                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                      _                                                                          +                                                                                                                                                                                                                                                                    I                                                                  8                                  ⁢                                  k                                                                                            ⁢                                                              ρ                                                                                                      8                                    ⁢                                    k                                                                    -                                  1                                                                                                                      _                                                    ⁢                                                      η                                                                                          8                                ⁢                                k                                                            -                              1                                                                                                      +                                                                                                            Q                                                              8                                ⁢                                k                                                                                      ⁢                                                          ρ                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                      ⁢                                                          η                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                                              _                                                                                                                                                                                                              ρ                                                            8                      ⁢                      k                                        +                    1                                                  =                                                                                                                                                          Q                                                                                          8                                ⁢                                k                                                            +                              1                                                                                ⁢                                                      ρ                                                          8                              ⁢                              k                                                                                ⁢                                                      η                                                          8                              ⁢                              k                                                                                                      +                                                                              I                                                                                          8                                ⁢                                k                                                            +                              1                                                                                ⁢                                                      ρ                                                          8                              ⁢                              k                                                                                ⁢                                                                                    η                                                              8                                ⁢                                k                                                                                      _                                                                          +                                                                                                                                                                                                                                                                    I                                                                                                      8                                    ⁢                                    k                                                                    +                                  1                                                                                            ⁢                                                              ρ                                                                  8                                  ⁢                                  k                                                                                                                      _                                                    ⁢                                                      η                                                          8                              ⁢                              k                                                                                                      +                                                                                                            Q                                                                                                8                                  ⁢                                  k                                                                +                                1                                                                                      ⁢                                                          ρ                                                              8                                ⁢                                k                                                                                      ⁢                                                          η                                                              8                                ⁢                                k                                                                                                              _                                                                                                                                                                            ⋮                                                                                            ρ                                                            8                      ⁢                      k                                        +                    7                                                  =                                                                                                                                                          Q                                                                                          8                                ⁢                                k                                                            +                              7                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            +                              6                                                                                ⁢                                                      η                                                                                          8                                ⁢                                k                                                            +                              6                                                                                                      +                                                                              I                                                                                          8                                ⁢                                k                                                            +                              7                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            +                              6                                                                                ⁢                                                                                    η                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                      _                                                                          +                                                                                                                                                                                                                                                                    I                                                                                                      8                                    ⁢                                    k                                                                    +                                  7                                                                                            ⁢                                                              ρ                                                                                                      8                                    ⁢                                    k                                                                    +                                  6                                                                                                                      _                                                    ⁢                                                      η                                                                                          8                                ⁢                                k                                                            +                              6                                                                                                      +                                                                                                            Q                                                                                                8                                  ⁢                                  k                                                                +                                7                                                                                      ⁢                                                          ρ                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                      ⁢                                                          η                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                                              _                                                                                                                                                                                                              η                                      8                    ⁢                    k                                                  =                                                                                                                                                          I                                                          8                              ⁢                              k                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            -                              1                                                                                ⁢                                                      η                                                                                          8                                ⁢                                k                                                            -                              1                                                                                                      +                                                                                                            Q                                                              8                                ⁢                                k                                                                                      _                                                    ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            -                              1                                                                                ⁢                                                                                    η                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                      _                                                                          +                                                                                                                                                                                                      Q                                                          8                              ⁢                              k                                                                                ⁢                                                                                    ρ                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                      _                                                    ⁢                                                      η                                                                                          8                                ⁢                                k                                                            -                              1                                                                                                      +                                                                                                            I                                                              8                                ⁢                                k                                                                                      ⁢                                                          ρ                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                      ⁢                                                          η                                                                                                8                                  ⁢                                  k                                                                -                                1                                                                                                              _                                                                                                                                                                                                              η                                                            8                      ⁢                      k                                        +                    1                                                  =                                                                                                                                                          I                                                                                          8                                ⁢                                k                                                            +                              1                                                                                ⁢                                                      ρ                                                          8                              ⁢                              k                                                                                ⁢                                                      η                                                          8                              ⁢                              k                                                                                                      +                                                                                                            Q                                                                                                8                                  ⁢                                  k                                                                +                                1                                                                                      _                                                    ⁢                                                      ρ                                                          8                              ⁢                              k                                                                                ⁢                                                                                    η                                                              8                                ⁢                                k                                                                                      _                                                                          +                                                                                                                                                                                                      Q                                                                                          8                                ⁢                                k                                                            +                              1                                                                                ⁢                                                                                    ρ                                                              8                                ⁢                                k                                                                                      _                                                    ⁢                                                      η                                                          8                              ⁢                              k                                                                                                      +                                                                                                            I                                                                                                8                                  ⁢                                  k                                                                +                                1                                                                                      ⁢                                                          ρ                                                              8                                ⁢                                k                                                                                      ⁢                                                          η                                                              8                                ⁢                                k                                                                                                              _                                                                                                                                                                            ⋮                                                                                            η                                                            8                      ⁢                      k                                        +                    7                                                  =                                                                                                                                                          I                                                                                          8                                ⁢                                k                                                            +                              7                                                                                ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            +                              6                                                                                ⁢                                                      η                                                                                          8                                ⁢                                k                                                            +                              6                                                                                                      +                                                                                                            Q                                                                                                8                                  ⁢                                  k                                                                +                                7                                                                                      _                                                    ⁢                                                      ρ                                                                                          8                                ⁢                                k                                                            +                              6                                                                                ⁢                                                                                    η                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                      _                                                                          +                                                                                                                                                                                                      Q                                                                                          8                                ⁢                                k                                                            +                              7                                                                                ⁢                                                                                    ρ                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                      _                                                    ⁢                                                      η                                                                                          8                                ⁢                                k                                                            +                              6                                                                                                      +                                                                                                            I                                                                                                8                                  ⁢                                  k                                                                +                                7                                                                                      ⁢                                                          ρ                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                      ⁢                                                          η                                                                                                8                                  ⁢                                  k                                                                +                                6                                                                                                              _                                                                                                                                                        }                            (        3        )            
The number n of precoders making up the parallel precoder unit 4 is changed depending on the data volumes of signals to be handled (the bit count of data) and the operation clock, and in the case where the data volumes are large, timing margins cannot be ensured.
In each of the first to eighth precoders 4-1 through 4-8, up to four logic circuits (AND circuits and OR circuits) are cascade-connected, as illustrated in FIG. 2. If the number n of precoders making up the parallel precoder unit 4 becomes large (for example, n=32), up to n×4 logic circuits are cascade-connected, which poses the problem that timing margins cannot be ensured.
In general, in order to ensure timing margins, cascade-connected logic circuits are divided, and operations are processed in multiple cycles while re-timing is performed by flip-flops. However, according to structural limitations of the precoder circuit, the operations of the one-cycle preceding modulated signals ρ8k+7 and η8k+7 and the current data I8k and Q8k have to be performed in one cycle, and therefore, the above-mentioned method of processing the operations in multiple cycles cannot be applied.